1. Field of the Invention
The present invention relates to an inverter control system which controls switching of power supply from a backup power source to an inverter.
2. Description of the Prior Art
The conventional control system for this purpose is shown in FIG. 1, in which the main circuit consists of an inverter 1, an a.c. reactor 2, a capacitor 3, thyristor a.c. switches 4 and 5, a backup power source 6, a reactor 7 for the backup power source, and a load 8. The main circuit is controlled by a PLL circuit made up of Schmitt trigger circuits 11 and 12, a phase comparator 13, a signal averaging resistor-capacitor pair 14 and 15, an operational amplifier 17 with its associated input resistor 16 and integrating capacitor 18, a voltage-controlled oscillator 19, and a voltage control circuit 20. Symbol V.sub.1 indicates the inverter voltage, I.sub.1 indicates the inverter current, V.sub.2 and I.sub.2 indicate the voltage and current of the backup power source 6, and V.sub.L indicates the load voltage.
In the above arrangement, the inverter 1 normally supplies power to the load 8, and in order for the load 8 in the event of failure to be switched to the backup power source 6 without a momentary power break, it is controlled by the PLL circuit to operate synchronously with the backup power source 6. The switching operation from the state of backup power supply to the normal state of power supply by the inverter 1 will be described with reference to FIG. 2. In switching a rated power supply of the load 8 from the backup power source 6 to the inverter 1 without a momentary power break, when the inverter switch 4 is turned on and the backup power source switch 5 is turned off at a time point t.sub.1, the inverter circuit I.sub.1 which makes a full transition of 0 to 100% causes a large transient variation in the inverter voltage, i.e., load voltage V.sub.L, due to the impedance of the a.c. reactor 2, as shown in the figure, and this is a major drawback of the conventional switching circuit.